Man-made and natural structures such as bridges, buildings, roads, parking garages, amusement park rides, hills, and the ground can move as a result of many influences. A structure can, for example, move from changes in its load (such as increases or decreases in the number of occupants, cars, trains, etc.), externally-induced vibration (such as vibration from local traffic, construction, earthquakes, wind, etc.), and other factors. If motions remain within a structure's design parameters, the structure is not likely to be at risk or a safety hazard. However, structures are at the mercy of their environment, and natural and unnatural influences have the potential for exerting more force on the structure than it is safely able to withstand. Also, a structure's ability to withstand forces may deteriorate from age, inadequate maintenance, or improper modification. Identifying damaging environmental inputs and unacceptable structural responses is critical for maintaining the safety and viability of the structure.
An accelerometer may be used to detect vibrations experienced by a structure, but accelerometers do not reveal whether the structure is tilting. An inclinometer may be used to detect changes in tilt, but they are designed to measure fixed angles that are maintained or constant and are thus not well-suited for dynamic environments. Inclinometers have a low sampling rate because they are designed to measure the low-frequency phenomenon of tilt with respect to gravity. Neither an accelerometer nor an inclinometer can by itself accurately discern motion from tilt. Acceleration signals may show up as artificial tilt, and tilt may show up as partial acceleration. For applications demanding precision, knowing what is inclination and what is acceleration can make a significant difference. When a building responds to a dynamic input, the responses are complex motions that have vibrations, inclinations, displacements, and roll components intermingled with each other and masking one another, making it very difficult to ascertain actual responses.
Also, inclinometers are sensitive, high-precision devices intended for relatively quiet environments, and they are consequently susceptible to damage in extreme environments. But by the time an unacceptable condition is observed using an inclinometer, it is usually too late to power down the inclinometer to prevent damage.
One representative dynamic environment is railway bridges, which are “live load” structures that carry more weight than their mass. That is, the weight of trains and train cars that traverse railway bridges often weigh many more times than the railway bridge itself. As a result, the bridge elements (spans, piers, and columns) move and vibrate tremendously when carrying the weight of trains. Such motions and vibrations cause conventional inclinometers to misread, and expose inclinometers to motions beyond their operational limits. As a consequence of such compromised performance, bridge engineers are not able to get a true reading during loading, and often are left with damaged sensor elements after loading.
What is needed is a monitoring method and system that can reliably monitor the overall integrity of a structure by overcoming these and other shortcomings.